• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 49 subscribers
  • Views 693 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LOG114 single supply operation

Murat Tumer
Prodigy 40 points
Other Parts Discussed in Thread: LOG114

Hello. I want to operate LOG114 from single supply. The application circuit in Figure 10 of the datasheet shows the cathodes of the photodiodes to be connected to GND, where Vcom is connected to Vref. Is this a correct connection? Should not the cathodes be connected to Vref at least, the potential of the anode side? In the current configuration, the photodiodes are forward biased.

A second question is, is there a need to bypass the Vref pin with a capacitor? This is not mentioned in the datasheet. 

Thanks in advance.

  • 0
    TI__Guru* 93105 points

    Hello Murat,

    The photodiode shown in the LOG114 single-supply figure 2, has 2.5 V applied to the diode cathode. This voltage is also applied to the non-inverting inputs of A1 and A2 as a common-mode voltage. This voltage is present at the A2 input, plus or minus the amplifier's voltage offset, and is applied to the diode's anode. Therefore, the photodiode has a very small voltage across it equal to the voltage offset which is very close to 0 V. Photodiodes are biased in use with 0 V, or a reverse bias voltage, across them. The photon generated current flows though the diode much like a reverse leakage current - against the diode arrow.

    Regarding a VREF a bypass capacitor it doesn't appear to be necessary. The reference has a low impedance output. I do not see one being applied in any of the LOG114 data sheet applications, or being used in the LOG114 EVM circuit.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Hello Thomas,

    Thank you for the detailed answer. Then, Fig. 2 is a more correct application circuit and I also think, Fig. 10 on the datasheet should be corrected.

    Kind regards,

    Murat